Modified film-forming polyurethane compositions are widely employed as protective coatings for substrates in service conditions where rigorous film performance characteristics are required. Polyurethane coatings are known to have the advantages of high gloss, and can resist abrasion, water and many chemicals. When formulated optimally, they can display high flexibility, impact resistance and toughness.
In the past, organic solvent-borne, moisture cure urethanes or one- and two-part solution polyurethane coatings were the norm for these applications. Typical commercially available products of this type are Tennant #420 and Tennant #4700 (Tennant Co., Minneapolis, Minn.). Likewise, epoxy coating systems have been available in both organic solvents and water, which can provide some unique characteristics, such as high chemical resistance. Typical of a commercially available water-based epoxy coating system is Tennant #405.
A number of techniques have been developed for crosslinking polyurethanes by introducing free amino groups into the polyurethane and then reacting the amino groups with epoxide crosslinking agents, or "polyepoxides." The final products are aqueous film-forming emulsions which are useful to form highly adherent coatings on a wide variety of substrates, such as metal, concrete, glass and the like. However, the reactants, solvent systems and stoichiometry of the functional groups involved can be critical to the spectrum of properties which is observed in the final crosslinked or "cured" coating.
Generally, an isocyanate-terminated urethane oligomer or urethane "prepolymer resin," is first prepared by reacting a polyhydroxyl compound and an organic polyisocyanate, wherein said polyisocyanate is used in an amount in excess of the stoichiometic equivalent amount. The urethane oligomer is then terminated with protected primary amino groups by reaction of all or a part of the free isocyanate groups (NCO) on the prepolymer with a compound comprising a substituent having a free, NCO-reactive hydrogen atom, e.g., a secondary amino group (R.sub.2 NH). The compound also contains one or more primary amino groups that are blocked with a removable protecting group, such as a dialkylketone. The protecting group is then removed, and the amino-terminated urethane oligomer is crosslinked, in aqueous emulsion, by reaction of the free amino groups with the epoxy groups of a polyepoxide crosslinking agent.
More specifically, A. J. Tortorello et al. (U.S. Pat. Nos. 4,427,804 and 4,489,179) disclose forming an isocyanate-terminated prepolymer by reacting diphenylmethane diisocyanate with polytetraoxymethylene glycol, which is in turn reacted with diethylenetriamine, in which the primary amino groups (NH.sub.2) were first protected by reaction with methyl isobutyl ketone (MIBK). The protected-primary amino group-containing prepolymer resin is then fully deprotected by dispersing in it aqueous acetic acid and then is reacted with a stoichiometric proportion of the diglycidyl ether of bisphenol A ("Epon 828"), to yield an epoxy crosslinked urethane coating composition. This methodology requires that the protecting agent is a highly hindered ketone.
H. Ohmura et al. (U.S. Pat. No. 4,190,567) discloses a curable, film-forming cationic polyurethane emulsion which is formed by mixing a neat polyepoxide crosslinking agent with an aqueous dispersion of an amine-terminated urethane prepolymer. The amino-terminated prepolymer is in turn formed by reacting the isocyanate-terminated prepolymer with diethylenetriame in methyl ethyl ketone (MEK). The epoxide is added to a "cationic polyurethane aqueous emulsion" formed by complete "neutralization" of the amine-terminated prepolymer with aqueous glycolic acid, residual MEK is stripped, and the product is diluted with water prior to use.
Commonly-assigned G. D. Ernst et al. (U.S. Pat. Nos. 4,816,825 and 4,772,643) disclose two-component aqueous urethane-epoxy systems that are substantially free of hazardous volatile organic solvents and free diisocyanate monomers, while retaining the desirable properties of a one-component, room temperature curable, solvent-borne coatings system. A typical system comprises, separately packaged, an aqueous dispersion of the polyepoxide component (e.g., Epon 828) and an aqueous dispersion of an amine-terminated urethane oligomer which has been dispersed in water with an amount of a volatile organic acid effective to neutralize only 25-40% of the amine groups, i.e., the amine groups are only partially "quaternized" or converted into the corresponding acid salt. An advantageous feature of this invention is the extended pot life of the finished product after the two aqueous components of the system are mixed at the application site, from no more than about 24 hours for prior systems, to as long as 3-4 days for the two-component aqueous emulsion system.
However, while exhibiting low toxicity and high stability, this composition is disadvantageous in that the components must be separately formulated, maintained, and then mixed together at some point by the end user. This can lead to increased packaging and storage costs, as well as to potential errors in formulation.
Therefore, a continuing need exists for improved ready-to-use urethane epoxy coating compositions.